Various modern-day telecommunications switching systems use a time-division multiplexed (TDM) communications medium as their switching fabric. An illustrative example of such systems is the Lucent Technologies Inc. Definity.RTM. private branch exchange (PBX). Port circuits that interface communications links and devices (applications) to the TDM medium normally provide access to only a relatively small number of the medium's time slots. For example, the port circuit packs of the Definity PBX provide access to any 32 time slots of the PBX's 256 time-slot TDM bus. Applications that need access to more than 32 time slots either use multiple port circuit packs or custom (and generally complex and expensive) time-slot interchange (TSI) circuits.
Applications that require access to more time slots than conventional port circuits provide, are becoming ever-more common. If such applications use multiple port circuit packs, they consume multiple circuit-pack slots on the TDM medium and thereby reduce the number of applications that the switching system can serve. If such applications use custom complex and expensive TDM interface circuits, they significantly increase the system's cost.
An ideal TDM interface would provide each application served by a port circuit pack access to all of the TDM medium's time slots that the application requires, in a simple, low-cost, manner.